RDF is a widely-used language that was originally developed for representing information (metadata) about resources in the World Wide Web. It may, however, be used for representing information about absolutely anything. When information has been specified using the generic RDF format, it may be consumed automatically by a diverse set of applications.
There are two standard vocabularies defined on RDF: RDF Schema (RDFS) and the Web Ontology Language (OWL). These vocabularies introduce RDF terms that have special semantics in those vocabularies. For simplicity, in the rest of the document, our use of the term RDF will also implicitly include RDFS and OWL. For more information and for a specification of RDF, see RDF Vocabulary Description Language 1.0: RDF Schema, available at www.w3.org/TR/rdf-schema/, OWL Web Ontology Language Overview, available at www.w3.org/TR/owl-features/, and Frank Manola and Eric Miller, RDF Primer, published by W3C and available in Sept., 2004 at www.w3.org/TR/rdf-primer/. The RDF Vocabulary Description Language 1.0: RDF Schema, OWL Web Ontology Language Overview, and RDF Primer are hereby incorporated by reference into the present patent application.
Facts in RDF are represented by RDF triples. Each RDF triple represents a fact and is made up of three parts, a subject, a predicate (sometimes termed a property), and an object. For example, the fact represented by the English sentence “John is 24 years old” can be represented in RDF by the subject, predicate, object triple <'John', ‘age’, ‘24’>, with ‘John’ being the subject, ‘age’ being the predicate, and ‘24’ being the object. In the following discussion, the values in RDF triples are termed lexical values.
With RDF, the values of predicates must ultimately resolve to lexical values termed universal resource identifiers (URIs), and the values of subjects must ultimately resolve to lexical values termed URIs and blank nodes. A URI is a standardized format for representing resources on the Internet, as described in RFD 2396: Uniform Resource Identifiers (URI): Generic Syntax, www.ietf.org/rfc/rfc2396.txt. RFD 2396 is hereby incorporated by reference into the present patent application. In the triples, the lexical values for the object parts may be literal values. In RDF, literal values are strings of characters, and can be either plain literals (such as “Immune Disorder”) or typed literals (such “2.4”^^ Axsd:decimal). The interpretations given to the lexical values in the members of the triple are determined by the application that is consuming it. For a complete description of RDF, see Frank Manola and Eric Miller, RDF Primer, published by W3C and available in September 2004 at www.w3.org/TR/rdf-primer/. The RDF Primer is hereby incorporated by reference into the present patent application.
In contrast to the URI approach of RDF data, relational database management systems (RDBMSs) store information in tables, where each piece of data is stored at a particular row and column. Information in a given row generally is associated with a particular object, and information in a given column generally relates to a particular category of information. For example, each row of a table may correspond to a particular employee, and the various columns of the table may correspond to employee names, employee social security numbers, and employee salaries. A user retrieves information from and makes updates to a relational database by interacting with a RDBMS application. Queries that are submitted to the RDBMS server must conform to the syntactical rules of a database query language, where one popular database query language, known as the Structured Query Language (SQL), provides users a variety of ways to specify information to be retrieved from relational tables.
Relational-based systems are the most common commercially available database systems now being used today. As such, there is a deep pool of existing relational-based tools and products that are now owned or used by organizations and individuals to access and analyze the relational data. However, because these tools are designed to work with relational-based data, such tools cannot be used to directly access and analyze the RDF-based data. The problem is that more and more data are being placed into RDF-based databases everyday. For example, RDF/OWL repositories are increasingly being created by government agencies, e.g., Data.gov, SNOMED, and DBPedia.
Therefore, there is a need for an improved approach for allowing integrated access to RDF-based data from relational-based tools.